Thixomolding material, method for manufacturing thixomolding material, and thixomolded article

ABSTRACT

A thixomolding material includes: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains SiO2 particles containing SiO2 as a main component. An average particle diameter of the SiO2 particles is less than 20.0 μm, and a mass fraction of the SiO2 particles in a total mass of the metal body and the SiO2 particles is 1.0 mass % or more and 40.0 mass % or less. The binder may contain waxes. A content of the binder may be 0.001 mass % or more and 0.200 mass % or less.

The present application is based on, and claims priority from JP Application Serial Number 2021-056784, filed Mar. 30, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a thixomolding material, a method for manufacturing a thixomolding material, and a thixomolded article.

2. Related Art

Magnesium has properties such as a low specific gravity, a good electromagnetic wave shielding property, good vibration damping capability, good machinability, and good biosafety. Based on such a background, parts made of magnesium alloys are beginning to be used in products such as automobiles, aircraft, mobile phones, and notebook computers.

For example, WO 03/069001 discloses a method for manufacturing a magnesium-based composite material including: mixing a first material containing magnesium and a second material containing a SiO₂ component to obtain a mixture; filling a mold with the mixture to obtain a powder compact; and heating the powder compact to react magnesium with SiO₂.

WO 2004/062837 discloses a method for manufacturing a magnesium-based composite material including: powder-solidifying a magnesium composite powder in which a silica powder is adhered to surfaces of coarse magnesium alloy particles; heating the obtained solidified body to generate compound particles by solid-phase reaction synthesis in the solidified body; and densifying the solidified body in which the compound particles are generated.

In the magnesium-based composite materials thus manufactured, by adding SiO₂ (silica), it is possible to improve strength, abrasion resistance, and the like of a magnesium alloy.

Such manufacturing methods described in WO 03/069001 and WO 2004/062837 involve a process of compressing the mixture or the magnesium composite powder for molding. Therefore, there is a problem that a shape of a molded article to be manufactured is restricted and a degree of freedom in shape is low. In addition, in these methods, since interfaces between magnesium and SiO₂ are interfaces between solids, contact stability between the interfaces is poor. Therefore, in these methods, since reactivity between magnesium and SiO₂ is low, and a magnesium alloy cannot be sufficiently improved, there is a problem that mechanical strength and rigidity of the molded article to be manufactured are insufficient.

SUMMARY

A thixomolding material according to an application example of the present disclosure includes: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains SiO₂ particles containing SiO₂ as a main component. An average particle diameter of the SiO₂ particles is less than 20.0 μm, and a mass fraction of the SiO₂ particles in a total mass of the metal body and the SiO₂ particles is 1.0 mass % or more and 40.0 mass % or less.

A method for manufacturing a thixomolding material according to an application example of the present disclosure includes: a preparation step of preparing a mixture containing a metal body containing Mg as a main component, SiO₂ particles containing SiO₂ as a main component, a binder, and a solvent; a stirring step of stirring the mixture; and a debindering step of removing, by heating the stirred mixture, at least a part of the binder contained in the mixture to obtain a thixomolding material. A mass fraction of the SiO₂ particles in a total mass of the metal body and the SiO₂ particles is 1.0 mass % or more and 40.0 mass % or less, and a content of the binder in the thixomolding material is 0.001 mass % or more and 0.200 mass % or less.

A thixomolded article according to an application example of the present disclosure includes: a matrix portion containing Mg as a main component; a first particle portion that is dispersed in the matrix portion and contains Mg₂Si as a main component; and a second particle portion that is dispersed in the matrix portion, present independently of the first particle portion, and contains MgO as a main component. When observing a cross section, a total area fraction of the first particle portion and the second particle portion, in a range of 500 μm square centered on a point at a depth of 1 mm from a surface, is 1.0% or more and 55.0% or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of an injection molding machine used for a thixomolding method.

FIG. 2 is a cross-sectional view schematically showing a thixomolding material according to an embodiment.

FIG. 3 is a process diagram illustrating a method for manufacturing the thixomolding material according to the embodiment.

FIG. 4 is a partial cross-sectional view schematically showing a thixomolded article according to the embodiment.

FIG. 5 is an example of an X-ray diffraction pattern obtained for the thixomolded article according to the embodiment.

FIG. 6 is an observation image (secondary electron image) obtained when a cut surface of the thixomolded article corresponding to Examples is observed with a scanning electron microscope.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a thixomolding material, a method for manufacturing a thixomolding material, and a thixomolded article according to the present disclosure will be described in detail based on embodiments illustrated in the accompanying drawings.

1. Thixomolding Method

First, a thixomolding method using a thixomolding material according to an embodiment will be described.

The thixomolding method is a molding method in which a pellet-like or chip-like material is heated in a cylinder to bring the material into a solid-liquid coexistence state in which a liquid phase and a solid phase coexist, then thixotropy is developed by rotation of a screw, and the obtained semi-solidified product is injected into a mold. According to such a thixomolding method, since fluidity of the semi-solidified product is enhanced by heating and shearing, a part having a small thickness or a part having a complicated shape can be molded as compared with, for example, a die casting method.

FIG. 1 is a cross-sectional view showing an example of an injection molding machine used for the thixomolding method.

As shown in FIG. 1, an injection molding machine 1 includes a mold 2, a hopper 5, a heating cylinder 7, a screw 8, and a nozzle 9. In the mold 2, a cavity Cv is formed. When a thixomolding material 10 is charged into the hopper 5, the thixomolding material 10 is supplied to the heating cylinder 7. The thixomolding material 10 supplied to the heating cylinder 7 is transferred while being heated by a heater 6 and being sheared by the screw 8. Accordingly, the thixomolding material 10 is semi-melted and slurried. The obtained slurry is injected into the cavity Cv in the mold 2 through the nozzle 9 without being exposed to the atmosphere. Then, the slurry injected into the cavity Cv is cooled to obtain a thixomolded article.

The hopper 5 may be charged with other materials together with the thixomolding material 10.

2. Thixomolding Material

Next, a thixomolding material according to an embodiment will be described.

FIG. 2 is a cross-sectional view schematically showing the thixomolding material according to the embodiment.

The thixomolding material 10 shown in FIG. 2 is a raw material to be used in the thixomolding method, and includes a chip-like metal body 11, a coating portion 12 that adheres to the surface of the metal body 11, and an adhesive portion 13 that contains a binder and adheres the metal body 11 to the coating portion 12.

2.1. Metal Body

The metal body 11 is, for example, a section obtained by machining or cutting an Mg-based alloy cast with a mold or the like. A method for manufacturing the metal body 11 is not limited thereto.

The metal body 11 contains Mg as a main component and contains various additive components. Examples of the additive components include lithium, beryllium, calcium, aluminum, silicon, manganese, iron, nickel, copper, zinc, strontium, yttrium, zirconium, silver, tin, gold, and rare earth elements, and a mixture of one or more of the additive components is used. Examples of the rare earth elements include cerium.

The main component refers to an element having the highest content in the metal body 11. The content of the main component is preferably more than 50 mass %, more preferably 70 mass % or more, and still more preferably 80 mass % or more.

The additive components preferably include aluminum and zinc. Accordingly, the melting point of the metal body 11 is lowered, and the fluidity of the slurry is improved. As a result, moldability of the thixomolding material 10 can be enhanced.

In addition, the additive components may include at least one selected from the group consisting of manganese, yttrium, strontium, and rare earth elements in addition to aluminum and zinc. Accordingly, mechanical properties, corrosion resistance, abrasion resistance, and thermal conductivity of the thixomolded article can be enhanced.

The additive components may be present in a form of a simple substance, an alloy, an oxide, an intermetallic compound, and the like in the metal body 11. In addition, the additive components may be segregated or uniformly dispersed in a crystal grain boundary of a metal structure such as Mg or an Mg alloy in the metal body 11.

The average particle diameter of the thixomolding material 10 is not particularly limited, and is preferably 0.5 mm or more, and more preferably 1.5 mm or more and 10.0 mm or less. By setting the average particle diameter within the above range, generation of bridges and the like in the heating cylinder 7 of the injection molding machine 1 can be prevented.

The average particle diameter of the thixomolding material 10 is an average value of diameters of circles having the same area as a projected area of the thixomolding material 10. The average value is calculated based on 100 or more thixomolding materials 10 selected at random.

An average aspect ratio of the thixomolding material 10 is preferably 5.0 or less, and more preferably 4.0 or less. In the thixomolding material 10 having such an average aspect ratio, a filling property in the heating cylinder 7 is enhanced and temperature uniformity during heating is improved. As a result, a thixomolded article having high mechanical properties and high dimensional accuracy can be obtained.

The average aspect ratio of the thixomolding material 10 is an average value of aspect ratios calculated based on major axis/minor axis in a projection image of the thixomolding material 10. The average value is calculated based on 100 or more thixomolding materials 10 selected at random. The major axis is the maximum length that can be taken in the projection image, and the minor axis is the maximum length in the direction orthogonal to the major axis.

2.2. Coating Portion

The coating portion 12 contains SiO₂ particles 14 containing SiO₂ as a main component. Specifically, for example, a plurality of SiO₂ particles 14 are adhered to the surface of the metal body 11 to form the coating portion 12.

The coating portion 12 preferably covers the entire surface of the metal body 11, or may cover a part of the surface.

The SiO₂ particles 14 are not particularly limited as long as they are particles containing silicon oxide as a main component, and the SiO₂ particles 14 may be particles containing amorphous SiO₂ (silica glass) as a main component, or may be particles containing SiO₂ crystals (such as quartz) as a main component.

The average particle diameter of the SiO₂ particles 14 is less than 20.0 μm, preferably 0.5 μm or more and 10.0 μm or less, and more preferably 1.0 μm or more and 8.0 μm or less. By setting the average particle diameter of the SiO₂ particles 14 within the above range, the balance between a coverage of the coating portion 12 and the SiO₂ content in the thixomolding material 10 can be optimized. In addition, when the SiO₂ particles are adhered to the surface of the metal body 11, the SiO₂ particles can be uniformly distributed, and the SiO₂ particles 14 are less likely to fall off.

When the average particle diameter of the SiO₂ particles 14 is less than the above lower limit value, the SiO₂ particles 14 are less likely to be dispersed, and thus the above-described balance may be deteriorated. On the other hand, when the average particle diameter of the SiO₂ particles 14 is more than the above upper limit value, the SiO₂ particles 14 may easily fall off.

In the thixomolding material 10, the mass fraction of the SiO₂ particles 14 in the total mass of the metal body 11 and the SiO₂ particles 14 is 1.0 mass % or more and 40.0 mass % or less, preferably 1.5 mass % or more and 30.0 mass % or less, and more preferably 5.0 mass % or more and 20.0 mass % or less. By setting the mass fraction of the SiO₂ particles 14 within the above range, a decrease in moldability of the thixomolding material 10 can be prevented while preventing a large decrease in mechanical strength of the thixomolded article to be manufactured.

When the mass fraction of the SiO₂ particles 14 is less than the above lower limit value, the mechanical strength of the thixomolded article may not be sufficiently enhanced. On the other hand, when the mass fraction of the SiO₂ particles 14 is more than the above upper limit value, the moldability of the thixomolding material 10 may be deteriorated.

The coating portion 12 may contain a substance other than the SiO₂ particles 14. In this case, the content of the substance other than the SiO₂ particles 14 may be less than the content of the SiO₂ particles 14 in terms of mass ratio.

The SiO₂ particles 14 may contain an element other than Si and O. In this case, the content of the element other than Si and O may be less than the content of Si and less than the content of O in terms of mass ratio.

2.3. Adhesive Portion

The adhesive portion 13 is interposed between the metal body 11 and the SiO₂ particles 14 or between the SiO₂ particles 14.

The adhesive portion 13 contains a binder. As the binder, organic materials that bond the metal body 11 to the coating portion 12 are used. Examples of the organic materials include various resins, waxes, alcohols, higher fatty acids, fatty acid metals, higher fatty acid esters, higher fatty acid amides, nonionic surfactants, and silicone-based lubricants. The various resins include: polyolefins such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymers; acrylic resins such as polymethyl methacrylate and polybutyl methacrylate; styrene resins such as polystyrene; polyvinyl chloride; polyvinylidene chloride; polyamide; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyether; polyvinyl alcohol; polyvinyl pyrrolidone; or copolymers thereof. In addition, the binder may be a mixture containing at least one of these components and another component, or may be a mixture containing two or more of these components.

Among these, the binder preferably contains waxes, and more preferably contains paraffin wax or a derivative thereof. The waxes have a good binding property, and can strongly bond the metal body 11 to the SiO₂ particles 14 or strongly bond the SiO₂ particles 14 to each other. When using the waxes in combination with debindering conditions, it is possible to obtain a thixomolding material capable of reducing generation of gas during molding to a low level.

Examples of the waxes include natural waxes and synthetic waxes. The natural waxes include: plant waxes such as candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba oil; animal waxes such as beeswax, lanolin, and spermaceti; mineral waxes such as Montan wax, ozokerite, and ceresin; and petroleum waxes such as paraffin wax, microcrystalline wax, and petrolatum. The synthetic waxes include: modified waxes such as synthetic hydrocarbons such as polyethylene wax, Montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives; hydrogenated waxes such as hardened castor oil and hardened castor oil derivatives; fatty acids such as 12-hydroxystearic acid; acid amides such as stearamide; and ester such as phthalic anhydride ester.

As described above, the thixomolding material 10 according to the embodiment includes the metal body 11 and the coating portion 12. The metal body 11 contains Mg as a main component. The coating portion 12 adheres to the surface of the metal body 11 via the binder, and contains the SiO₂ particles 14 containing SiO₂ as a main component. The average particle diameter of the SiO₂ particles 14 is less than 20.0 μm. The mass fraction of the SiO₂ particles in the total mass of the metal body 11 and the SiO₂ particles 14 is 1.0 mass % or more and 40.0 mass % or less.

By performing thixomolding using such a thixomolding material 10, SiO₂ can be uniformly dispersed in a semi-molten state. Accordingly, in the thixomolding, Mg₂Si and MgO are precipitated while being uniformly dispersed in the course of solidification. As a result, a thixomolded article having high mechanical strength and high rigidity can be obtained.

The thixomolding material 10 may contain additives other than the metal body 11, the coating portion 12, and the adhesive portion 13 described above. Examples of the additives include a coupling agent, a surfactant, a dispersant, a lubricant, an antioxidant, an ultraviolet absorber, a thickener, a rust inhibitor, a preservative, and a fungicide.

3. Method for Manufacturing Thixomolding Material

Next, a method for manufacturing the above-mentioned thixomolding material 10 will be described.

FIG. 3 is a process diagram illustrating the method for manufacturing the thixomolding material according to the embodiment.

The method for manufacturing the thixomolding material 10 shown in FIG. 3 includes a preparation step S102, a drying step S104, a stirring step S106, and a debindering step S108.

3.1. Preparation Step

In the preparation step S102, a mixture containing a metal body 11, SiO₂ particles 14, a binder, and a solvent is prepared. The metal body 11 is similar to the above-described metal body 11. In addition, the SiO₂ particles 14 are similar to the above-described SiO₂ particles 14.

The solvent is not particularly limited as long as it is a liquid in which the binder is dispersed. Examples of the solvent include water, isopropanol, and acetone. For mixing, a mixer, a kneader, or the like is used. This step may be a step of preparing a mixture prepared in advance.

The content of the binder in the mixture is not particularly limited, and is preferably 1.0 mass % or more and 30.0 mass % or less, more preferably 2.0 mass % or more and 15.0 mass % or less, and still more preferably 3.0 mass % or more and 10.0 mass % or less. By setting the content of the binder within the above range, the SiO₂ particles 14 can be uniformly dispersed based on a dispersion action of the binder.

When the content of the binder is less than the above lower limit value, an amount of the binder is insufficient, and it may be difficult to uniformly adhere the SiO₂ particles 14 to the metal body 11, and it may be difficult to uniformly disperse the SiO₂ particles 14. On the other hand, when the content of the binder is more than the above upper limit value, the amount of the binder becomes excessive, and the SiO₂ particles 14 which are not adhered to the metal body 11 may easily aggregate, or an amount of the binder residue may increase in the debindering step S108 described later and an internal defect may easily occur in the thixomolded article.

The temperature of the solvent is preferably set to be equal to or higher than the melting point of the binder, if necessary. Accordingly, the binder is easily dissolved in the solvent. As a result, the binder can be dispersed more uniformly. The temperature of the solvent is preferably set to be higher than the melting point of the binder by 10° C. or more, and more preferably set to be higher by 20° C. or more and 50° C. or less.

In this case, the above-described mixture may be placed in a container, and the entire container may be heated from the outside using a hot bath or the like.

The melting point of the binder to be used is not particularly limited, and is preferably 40° C. or higher and 80° C. or lower, more preferably 43° C. or higher and 65° C. or lower, and still more preferably 45° C. or higher and 60° C. or lower. When the melting point of the binder is within the above range, the binder can be efficiently melted in a short time. In addition, when the melting point of the binder is within the above range, the thixomolding material 10 to be manufactured may have good lubricity in thixomolding, and can increase melt fluidity of the slurry.

3.2. Drying Step

In the drying step S104, the mixture is dried. Accordingly, the SiO₂ particles 14 are adhered to the surface of the metal body 11 via the binder, and the solvent is volatilized to obtain a dried body. In the present embodiment, since the SiO₂ particles 14 are dispersed using the binder, the SiO₂ particles can be adhered to the surface of the metal body 11 with a uniform thickness.

For drying, a method of heating the mixture, a method of exposing the mixture to a gas, or the like is used. Among these methods, when the mixture is heated, for example, the entire container containing the mixture may be heated using a hot bath or the like. In the drying step S104, the entire solvent in the mixture may be removed, or a part of the solvent may remain without being removed.

A temperature at which the mixture is heated may be equal to or higher than the temperature at which the solvent volatilizes and the binder softens, specifically, the temperature is set according to a composition of the solvent, and is preferably 40° C. or higher and 120° C. or lower, and more preferably 50° C. or higher and 80° C. or lower. Accordingly, the solvent can be volatilized and removed while preventing the SiO₂ particles 14 adhered to the surface of the metal body 11 from falling off.

In addition, a time for heating the mixture is appropriately set depending on the heating temperature, and is, for example, preferably 10 minutes or longer and 300 minutes or shorter, and more preferably 20 minutes or longer and 200 minutes or shorter.

The drying step S104 may be performed as necessary, and may be omitted, or the drying step S104 and the stirring step S106 may be performed at the same time.

3.3. Stirring Step

In the stirring step S106, the mixture is stirred. When the drying step is performed, the dried mixture is stirred. For stirring, a method using a stirring bar, a stirrer, or the like, a method of shaking a container containing a mixture with a lid, or the like is used. By such stirring, the SiO₂ particles 14 can be adhered to the surface of the metal body 11 via the binder. A part of the SiO₂ particles 14 may be directly adhered to the surface of the metal body 11 without interposing the binder. In addition, by stirring, formation of a block by aggregation of metal bodies 11 can be prevented.

After the stirring step S106, the drying step S104 and the stirring step S106 may be repeated as necessary. Accordingly, since the SiO₂ particles 14 are repeatedly adhered, the SiO₂ particles 14 can be adhered to the surface of the metal body 11 in multiple layers. As a result, more SiO₂ particles 14 can be adhered to the surface of the metal body 11. The number of repetitions is not particularly limited, and is, for example, 2 or more and 10 or less. Also in this case, the drying step S104 and the stirring step S106 may be performed at the same time.

3.4. Debindering Step

In the debindering step S108, a debindering treatment is performed on the stirred mixture. Accordingly, the thixomolding material 10 is obtained. Examples of the debindering treatment include a method of heating the mixture, and a method of exposing the mixture to a gas for decomposing the binder. Accordingly, at least a part of the binder contained in the mixture can be removed. As a result, by preventing a large amount of binder from being transferred into the heating cylinder 7, generation of a large amount of gas in the heating cylinder 7 can be prevented.

A heating temperature for the mixture in the debindering treatment is not particularly limited as long as it is a temperature at which the binder is thermally decomposed, and the heating temperature is preferably 200° C. or higher and 500° C. or lower, and more preferably 250° C. or higher and 450° C. or lower. By setting the heating temperature within the above range, the binder can be appropriately removed while preventing an adverse effect on the metal body 11 due to the debindering treatment.

When the heating temperature is lower than the above lower limit value, a large amount of binder which is not removed remains, and a large amount of gas may be generated in the heating cylinder 7. On the other hand, when the heating temperature is higher than the above upper limit value, there is a concern that an adverse effect due to heat may occur on the metal body 11, or the binder may be completely removed and the SiO₂ particles 14 may fall off from the metal body 11.

A heating time for the mixture in the debindering treatment is not particularly limited, and may be, for example, 5 minutes or longer, and is preferably 1 hour or longer and 100 hours or shorter, and more preferably 10 hours or longer and 50 hours or shorter. Accordingly, the binder can be appropriately removed while preventing an adverse effect on the metal body 11 due to the debindering treatment.

An amount of the binder, that is, a content of the binder in the thixomolding material 10 after debindering is not particularly limited, and is preferably 0.001 mass % or more and 0.200 mass % or less, more preferably 0.010 mass % or more and 0.100 mass % or less, and still more preferably 0.015 mass % or more and 0.040 mass % or less. By setting the content of the binder in the thixomolding material 10 within the above range, the amount of the binder to be thermally decomposed in the heating cylinder 7 can be prevented from increasing more than necessary while ensuring adhesiveness of the coating portion 12 realized by the adhesive portion 13.

When the content of the binder is less than the above lower limit value, the amount of the binder is insufficient, and the coating portion 12 may easily fall off. On the other hand, when the content of the binder is more than the above upper limit value, the amount of the binder becomes excessive, a large amount of gas is generated in the heating cylinder 7, and voids may be easily generated in the thixomolded article.

As described above, the method for manufacturing the thixomolding material 10 according to the present embodiment includes the preparation step S102, the stirring step S106, and the debindering step S108. In the preparation step S102, the mixture containing the metal body 11 containing Mg as a main component, the SiO₂ particles 14 containing SiO₂ as a main component, the binder, and the solvent is prepared. In the stirring step S106, the mixture is stirred. In the debindering step S108, the stirred mixture is heated to remove at least a part of the binder contained in the mixture, thereby obtaining the thixomolding material 10. The mass fraction of the SiO₂ particles 14 in the total mass of the metal body 11 and the SiO₂ particles 14 is 1.0 mass % or more and 40.0 mass % or less. The content of the binder in the thixomolding material 10 is 0.001 mass % or more and 0.200 mass % or less.

According to such a configuration, even when the amount of the SiO₂ particles 14 is large, the SiO₂ particles 14 can be adhered to the surface of the metal body 11 via the binder, and thus the SiO₂ particles 14 can be uniformly dispersed in the heating cylinder 7. Accordingly, a thixomolded article in which reaction chance between Mg and SiO₂ is uniformly ensured, and Mg₂Si and MgO are uniformly dispersed and precipitated in the course of solidification can be manufactured. Then, Mg₂Si and MgO function as fillers, and an Mg crystal precipitated in the course of solidification can be micronized. As a result, a thixomolded article having high mechanical strength and high rigidity can be obtained.

The thixomolding material 10 does not necessarily have to be manufactured by this manufacturing method. That is, the thixomolding material 10 may be manufactured, for example, without going through the debindering step S108.

4. Thixomolded Article

Next, a thixomolded article according to the embodiment will be described.

FIG. 4 is a partial cross-sectional view schematically showing the thixomolded article according to the embodiment.

The thixomolded article 100 shown in FIG. 4 is a molded article obtained by a thixomolding method, and includes a matrix portion 200, a first particle portion 300, and a second particle portion 400. The matrix portion 200 is a portion mainly derived from the metal body 11 of the thixomolding material 10, and contains Mg as a main component. The first particle portion 300 is a portion mainly derived from a reaction product of the coating portion 12 of the thixomolding material 10 and Mg, and contains Mg₂Si as a main component. The second particle portion 400 is a portion mainly derived from a reaction product of the coating portion 12 of the thixomolding material 10 and Mg, and contains MgO as a main component. When observing a cross section of the thixomolded article 100, a total area fraction Ac of the first particle portion 300 and the second particle portion 400, in a range of 500 μm square centered on a point at a depth of 1 mm from a surface, is 1.0% or more and 55.0% or less.

As shown in FIG. 4, when viewing the cross section of the thixomolded article 100, an area occupied by the matrix portion 200 is larger than an area occupied by the first particle portion 300 and an area occupied by the second particle portion 400. Therefore, the first particle portion 300 and the second particle portion 400 are in a state of being dispersed in the matrix portion 200. In addition, the second particle portion 400 is present independently of the first particle portion 300.

An Mg₂Si simple substance and an MgO simple substance both have a Young's modulus higher than that of an Mg simple substance. Therefore, the first particle portion 300 and the second particle portion 400 function as fillers for enhancing rigidity. Therefore, the thixomolded article 100 including the first particle portion 300 and the second particle portion 400 has high rigidity.

In addition, the first particle portion 300 and the second particle portion 400 prevent coarsening of Mg crystals contained in the matrix portion 200. Therefore, in the matrix portion 200, the Mg crystals can be micronized. Accordingly, the thixomolded article 100 has high mechanical strength.

Further, the second particle portion 400 containing MgO as a main component also have a function of inhibiting dendritic or needle-like abnormal growth of the first particle portion 300 containing Mg₂Si as a main component. Due to this function, the first particle portion 300 has an isotropic shape, and is thus less likely to become a starting point of a crack or the like. Therefore, in the thixomolded article 100 including both the first particle portion 300 and the second particle portion 400, the mechanical strength is further enhanced.

Elemental analysis is used to identify the matrix portion 200, the first particle portion 300, and the second particle portion 400.

Examples of an elemental analysis method include: iron and steel-atomic absorption spectrometry defined in JIS G 1257:2000; iron and steel-ICP emission spectrometry defined in JIS G 1258:2007; iron and steel-spark discharge emission spectrometry defined in JIS G 1253:2002; iron and steel-X-ray fluorescence analysis defined in JIS G 1256:1997; and weight, titration, and absorption photometry defined in JIS G 1211 to G 1237.

FIG. 4 illustrates a range A1 of 500 μm square starting from a surface 101 of the thixomolded article 100, and a range A2 of 500 μm square centered on a point at a depth of 1 mm from the surface 101. When a total area fraction of the first particle portion 300 and the second particle portion 400 in the range A1 is defined as As [%] and a total area fraction of the first particle portion 300 and the second particle portion 400 in the range A2 is defined as Ac [%], |As−Ac|/Ac is preferably 50.0% or less, more preferably 40.0% or less, and still more preferably 20.0% or less in the thixomolded article 100.

In such a thixomolded article 100, a difference in occupied areas of the first particle portion 300 and the second particle portion 400 between the range A1 located in the vicinity of the surface 101 and the range A2 located at a deeper position is reduced. That is, in such a thixomolded article 100, uneven distribution of the first particle portion 300 and the second particle portion 400 is prevented. Accordingly, the rigidity of the thixomolded article 100 can be particularly enhanced.

The total area fraction As of the first particle portion 300 and the second particle portion 400 in the range A1 is calculated as follows. First, in an observation image of the range A1, a total area of the first particle portion 300 and the second particle portion 400 is calculated by image processing. For the image processing, for example, image analysis software OLYMPUS Stream or the like can be used. A magnification of the observation image is preferably 300 times or more. Next, a ratio of the total area of the first particle portion 300 and the second particle portion 400 to a total area of the range A1 is calculated. This ratio is the area fraction As. The range A1 is a range having a square shape with a side of 500 μm, and at least a part of the range A1 may be in contact with the surface 101.

The total area fraction Ac of the first particle portion 300 and the second particle portion 400 in the range A2 is also calculated in the same manner as the area fraction As.

The range A2 is a range having a square shape with a side of 500 μm, and a center point O of the range A2 is a point at a depth of 1 mm from the surface 101. When a length in a depth direction is less than 2 mm in the cross section of the thixomolded article 100, a midpoint of the length in the depth direction can be regarded as the center point O.

The area fraction As and the area fraction Ac are both determined by the content of SiO₂ in the thixomolded article 100, and are preferably 1.0% or more and 55.0% or less, more preferably 2.0% or more and 50.0% or less, and still more preferably 3.0% or more and 45.0% or less. When the area fraction As and the area fraction Ac are within the above range, an effect of enhancing the rigidity and the mechanical strength of the thixomolded article 100 are more remarkable.

In the ranges A1 and A2, the area fraction of the first particle portion 300 is defined as A(Mg₂Si), and the area fraction of the second particle portion 400 is defined as A(MgO).

In this case, an area fraction ratio A(Mg₂Si)/A(MgO) is preferably 0.5 or more and 3.0 or less, more preferably 1.0 or more and 2.5 or less, and still more preferably 1.1 or more and 2.0 or less. When the area fraction ratio is within the above range, a balance between the first particle portion 300 and the second particle portion 400 is good. Therefore, it is possible to achieve a function of inhibiting abnormal growth of the first particle portion 300 by the second particle portion 400 while ensuring a function of improving mechanical strength and rigidity by both the first particle portion 300 and the second particle portion 400.

An average particle diameter of the first particle portion 300 is defined as D(Mg₂Si), and an average particle diameter of the second particle portion 400 is defined as D(MgO). The average particle diameters D(Mg₂Si) and D(MgO) are preferably 0.5 μm or more and 10.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less. When the average particle diameters D(Mg₂Si) and D(MgO) are within the above range, the first particle portion 300 and the second particle portion 400 are less likely to become starting points of a crack or the like. Accordingly, mechanical strength such as bending strength and tensile strength of the thixomolded article 100 can be enhanced.

The average particle diameters D(Mg₂Si) and D(MgO) are calculated as follows. First, particle diameters of the first particle portion 300 and particle diameters of the second particle portion 400 included in the range A1 and the range A2 are all measured. The particle diameter of the first particle portion 300 is an intermediate value between a length of a major axis and a length of a minor axis in the observation image of the first particle portion 300. An average value of the particle diameters calculated in this manner is the average particle diameter of the first particle portion 300. The particle diameter of the second particle portion 400 is an intermediate value between a length of a major axis and a length of a minor axis in the observation image of the second particle portion 400. An average value of the particle diameters calculated in this manner is the average particle diameter of the second particle portion 400.

An average particle diameter ratio D(Mg₂Si)/D(MgO) is preferably 0.4 or more and 2.0 or less, more preferably 0.5 or more and 1.4 or less, and still more preferably 0.6 or more and 1.1 or less. When the average particle diameter ratio D(Mg₂Si)/D(MgO) is within the above range, both the function of improving the mechanical strength and rigidity by both the first particle portion 300 and the second particle portion 400 and the function of inhibiting the abnormal growth of the first particle portion 300 by the second particle portion 400 can be more favorably achieved.

Although not shown, the thixomolded article 100 may include a third particle portion containing SiO₂ as a main component. SiO₂ also has a Young's modulus higher than that of magnesium. Therefore, by providing the third particle portion, the rigidity of the thixomolded article 100 can be enhanced.

The tensile strength of the thixomolded article 100 is preferably 180 MPa or more and 300 MPa or less, and more preferably 190 MPa or more and 250 MPa or less. Further, the Young's modulus of the thixomolded article 100 is preferably 43 GPa or more and 80 GPa or less, and more preferably 48 GPa or more and 70 GPa or less.

The thixomolded article 100 having a tensile strength and a Young's modulus within the above ranges has particularly high specific strength and specific rigidity. Since such a thixomolded article 100 is lightweight and has high strength, and is thus suitable for, for example, parts used in a transportation device such as an automobile and an aircraft, parts used in a mobile device such as a mobile terminal and a notebook computer, and the like.

Further, 0.2% yield strength of the thixomolded article 100 is preferably 155 MPa or more and 300 MPa or less, and more preferably 165 MPa or more and 240 MPa or less.

The tensile strength of the thixomolded article 100 is measured as follows. First, a test piece is cut out from the thixomolded article 100. Examples of the test piece include a No. 13 test piece defined in JIS. Next, the test piece is attached to a tensile tester, and stress corresponding to a maximum force applied to the test piece at 25° C. is calculated. The obtained stress is defined as the tensile strength of the thixomolded article 100.

The Young's modulus of the thixomolded article 100 is measured as follows. First, a test piece is cut out from the thixomolded article 100. Next, the test piece is attached to a tensile tester, and a tensile load is applied to the test piece at 25° C. Next, an amount of change in tensile strain when the tensile load is varied and an amount of change in tensile stress when the tensile load is varied are respectively calculated. Then, a ratio of the latter amount of change to the former amount of change is calculated, and the obtained ratio is defined as the Young's modulus of the thixomolded article 100. The Young's modulus of the thixomolded article 100 may be a value measured by a method other than the above-mentioned measurement method, for example, a resonance method or an ultrasonic pulse method.

A Vickers hardness of the surface 101 of the thixomolded article 100 is preferably 75 or more and 200 or less, more preferably 80 or more and 120 or less, and still more preferably 85 or more and 100 or less.

When the Vickers hardness is within the above range, it is possible to obtain a thixomolded article 100 that has a high surface hardness and is less likely to be scratched.

The Vickers hardness of a surface 101 of the thixomolded article 100 is measured in accordance with a method of Vickers hardness test specified in JIS Z 2244:2009. A measurement load is 5 kgf.

FIG. 5 is an example of an X-ray diffraction pattern obtained for the thixomolded article according to the embodiment.

As shown in FIG. 5, when the thixomolded article 100 is subjected to a crystal structure analysis by an X-ray diffraction method, an X-ray diffraction pattern including a peak derived from α-Mg, a peak derived from β-Mg₁₇Al₁₂, a peak derived from Mg₂Si, and a peak derived from MgO is obtained. The α-Mg is a matrix phase of the matrix portion 200 and is a solid solution mainly containing Mg.

When a CuKα ray is used as the X-ray, a main peak derived from α-Mg is observed at 2θ=36.5° to 37.5°. The main peak derived from α-Mg refers to a peak having the maximum peak intensity among peaks derived from α-Mg. A main peak derived from β-Mg₁₇Al₁₂ is observed at 20=35.5° to 36.5°. The main peak derived from β-Mg₁₇Al₁₂ refers to a peak having the maximum peak intensity among peaks derived from β-Mg₁₇Al₁₂. A main peak derived from Mg₂Si is observed at 2θ=39.5° to 40.5°. The main peak derived from Mg₂Si refers to a peak having the maximum peak intensity among peaks derived from Mg₂Si. A main peak derived from MgO is observed at 2θ=42.5° to 43.5°. The main peak derived from MgO refers to a peak having the maximum peak intensity among peaks derived from MgO.

When the peak intensity of the main peak derived from α-Mg is taken as 100, the peak intensity of the main peak derived from Mg₂Si is preferably 5 or more and 100 or less, and more preferably 10 or more and 50 or less. In the thixomolded article 100 having such a peak intensity ratio, α-Mg and Mg₂Si are present in a well-balanced manner, so that both high rigidity and high strength are achieved.

When the peak intensity of the main peak derived from α-Mg is defined as 100, the peak intensity of the main peak derived from MgO is preferably smaller than the peak intensity of the main peak derived from Mg₂Si, and more preferably 10% or more and 50% or less of the peak intensity of the main peak derived from Mg₂Si. In the thixomolded article 100 having such a peak intensity ratio, α-Mg, Mg₂Si, and MgO are present in a well-balanced manner, so that both high rigidity and high strength are achieved.

The thixomolding material, the method for manufacturing the thixomolding material, and the thixomolded article according to the present disclosure are described above based on the illustrated embodiments. However, the thixomolding material and the thixomolded article according to the present disclosure are not limited to the above embodiment, and may be, for example, those obtained by adding any component to the above embodiment. The method for manufacturing the thixomolding material according to the present disclosure may be one obtained by adding any desired step to the above embodiment.

Examples

Next, specific examples of the present disclosure will be described.

5. Manufacturing of Thixomolding Material 5.1. Sample No. 1

First, a magnesium alloy chip as a metal body, SiO₂ particles, a binder, and a solvent were mixed to obtain a mixture. As the magnesium alloy chip, a chip of 4 mm×2 mm×1 mm made of an AZ91D alloy manufactured by STU, Inc. was used. The AZ91D alloy is an Mg-based alloy containing 9 mass % of Al and 1 mass % of Zn. In addition, as the binder, “Paraffin Wax 115” manufactured by Nippon Seiro Co., Ltd. was used. The melting point of the paraffin wax 115 was 48° C. Further, as the solvent, 35 mL of isopropanol was used per 4.5 g of the binder.

Next, the obtained mixture was heated to obtain a dried body. Subsequently, the obtained dried body was stirred. Thereafter, an operation of further heating the stirred dried body and then stirring the heated dried body was repeated three times. For stirring, a method of shaking a container containing the dried body was used.

Next, the stirred dried body was subjected to a debindering treatment. Accordingly, at least a part of the binder was removed to obtain a thixomolding material. In the obtained thixomolding material, almost the entire surface of the magnesium alloy chip was coated with the SiO₂ particles. Manufacturing conditions in the above manufacturing method are shown in Table 1. In Table 1, a charge amount of the SiO₂ particles is a ratio of a mass of the charged SiO₂ particles to a total mass of the magnesium alloy chip and the SiO₂ particles. A charge amount of the binder is a ratio of a mass of the charged binder to a mass of the entire thixomolding material.

5.2. Sample Nos. 2 to 5

Thixomolding materials were obtained in the same manner as in Sample No. 1 except that the manufacturing conditions were changed as shown in Table 1.

5.3. Sample No. 6

A thixomolding material was obtained in the same manner as in Sample No. 1 except that the SiO₂ particles and a binder were not used.

5.4. Sample Nos. 7 to 15

Thixomolding materials were obtained in the same manner as in Sample No. 1 except that the manufacturing conditions were changed as shown in Table 1. When the thixomolding material of Sample No. 14 was manufactured, the debindering treatment was omitted.

5.5. Sample No. 16

A thixomolding material was obtained in the same manner as in Sample No. 1, except that the SiO₂ particles were used, but the binder was not used.

In Table 1, among the thixomolding materials of the respective sample Nos., those corresponding to the present disclosure were referred to as “Examples,” and those not corresponding to the present disclosure were referred to as “Comparative Examples”.

6. Evaluation of Thixomolding Material

6.1. Amount of SiO₂ Particles after Debindering

For the thixomolding material of each sample No., an amount of the SiO₂ particles after debindering was calculated by the following method.

First, a mass M1 of the thixomolding material was measured. Since the thixomolding material is debindered, the remaining binder is regarded as substantially zero, and is not considered for calculation. Next, the thixomolding material was immersed in acetone and washed with an ultrasonic cleaner for 10 minutes. Accordingly, the adhered SiO₂ particles can be removed, and only the magnesium alloy chip can be taken out. Next, the magnesium alloy chip after washing was taken out from acetone, dried, and then a mass M2 was measured.

Then, a mass fraction of the SiO₂ particles with respect to the magnesium alloy chip calculated by (M1−M2)/M1×100 was defined as an amount [%] of the SiO₂ particles after debindering. Calculation results are shown in Table 1.

6.2. Adhesion Rate of SiO₂ Particles

An adhesion rate of the SiO₂ particles was calculated by dividing the amount of the SiO₂ particles after debindering by the charge amount of the SiO₂ particles. Calculation results are shown in Table 1.

6.3. Amount of Binder after Debindering

For the thixomolding material of each sample No., an amount of the binder after debindering was calculated by the following method.

First, a thermogravimetric change in a temperature range of 50° C. to 450° C. of one thixomolding material was measured by a differential thermogravimetric simultaneous measurement device (TGA/DSC1LF) manufactured by Mettler-Toledo. The temperature was increased at a temperature increase rate of 10° C./min while air was allowed to flow in at a flow rate of 30 mL/min in the atmosphere. Then, in order to eliminate an influence of the solvent, a weight change at 450° C., with reference to a weight at 200° C., was calculated as the amount of the binder after debindering. Calculation results are shown in Table 1.

TABLE 1 Evaluation result of thixomolding material Manufacturing condition for thixomolding materials SiO₂ particles Binder SiO₂ particles Debind- Amount Amount Average Binder Drying Number ering Debind- after after Example/ particle Charge Charge temp- Drying of temp- ering debind- Adhesion debind- Sample Comparative diameter amount amount erature time repetitions erature time ering rate ering No. Example μm mass% mass% ° C. min time(s) ° C. h mass % mass % mass % 1 Example 5 1.1 5 65 120 3 320 24 1.0 91 0.020 2 Example 5 2.1 8 65 120 3 400 24 2.0 95 0.018 3 Example 7 5.3 10 65 120 3 320 24 5.0 94 0.025 4 Example 8 12.0 10 65 120 3 320 24 10.4 87 0.029 5 Example 8 20.1 15 65 120 3 290 48 16.2 81 0.035 6 Comparative — 0.0 0 — — — — — — — — Example 7 Comparative 8 0.5 5 65 120 3 320 24 0.3 60 0.022 Example 8 Comparative 8 45.0 35 65 120 3 220 2 43.2 96 0.520 Example 9 Comparative 8 2.0 0.5 65 120 3 460 48 0.6 30 0.0005 Example 10 Comparative 25 5.3 10 65 120 3 320 24 0.5 9 0.024 Example 11 Example 2 5.0 5 65 120 3 320 24 4.0 80 0.021 12 Example 12 5.0 5 65 120 2 400 2 4.4 88 0.025 13 Example 8 5.0 10 65 120 0 250 12 3.2 64 0.035 14 Comparative 5 10.0 5 65 120 3 - - 8.0 80 4.2 Example 15 Comparative 8 2.0 5 65 120 0 220 0.5 1.6 80 0.560 Example 16 Comparative 5 10.0 0 — — — — — — — — Example

As shown in Table 1, it is confirmed that in the thixomolding materials corresponding to Examples, although the amount of the binder is reduced to the minimum by debindering, the SiO₂ particles are adhered at a sufficient adhesion rate.

7. Manufacturing of Thixomolded Article 7.1. Sample No. 17

The thixomolding material of Sample No. 1 was charged into an injection molding machine to obtain a thixomolded article of Sample No. 17. As the injection molding machine, a magnesium injection molding machine JLM75MG manufactured by The Japan Steel Works, Ltd. was used.

7.2. Sample Nos. 18 to 29

Thixomolded articles were obtained in the same manner as in Sample No. 17 except that the manufacturing conditions were changed as shown in Table 2.

8. Analysis of Thixomolded Article

The thixomolded article of each sample No. was cut, and the cut surface was observed with a scanning electron microscope. FIG. 6 is an observation image (secondary electron image) when the cut surface of the thixomolded article corresponding to Examples is observed with the scanning electron microscope. In FIG. 6, a first particle portion, a second particle portion and a matrix portion are observed.

In addition, the ranges A1 and A2 were specified, and the area fraction ratio A(Mg₂Si)/A(MgO), the average particle diameter ratio D(Mg₂Si)/D(MgO), the area fraction Ac, and |As−Ac|/Ac were calculated. Calculation results are shown in Table 2.

9. Evaluation of Thixomolded Article 9.1. Moldability

The thixomolded article of each sample No. was observed, and a molded state of the thixomolded article was evaluated based on melt fluidity, presence or absence of internal defects due to inclusion of blowholes and air, and the like. Specifically, those having many defects in melt fluidity and internal defects were evaluated as “NG,” and those having relatively few such defects were evaluated as “OK”. Evaluation results are shown in Table 2.

9.2. Tensile Strength

The tensile strength of the thixomolded article of each sample No. was measured. Specifically, a test piece conforming to JIS standard was formed from the thixomolded article, and the tensile strength was measured by a tensile tester. Measurement results are shown in Table 2.

9.3. Young's Modulus

The Young's modulus of the thixomolded article of each sample No. was measured. Measurement results are shown in Table 2.

9.4. Vickers Hardness

The Vickers hardness of the surface of the thixomolded article of each sample No. was measured. Measurement results are shown in Table 2.

TABLE 2 Manufacturing condition for thixomolded article and analysis result Evaluation result of thixomolded article Example/ Sample A(Mg₂Si)/ D(Mg₂Si)/ |As-Ac|/ Tensile Young's Vickers Sample Comparative No. of A(MgO) D(MgO) Ac Ac Moldability strength modulus hardness No. Example material — _— % _— — MPa GPa — 17 Example 1 0.8 0.6 2.3 38 OK 195 43 86 18 Example 2 1.4 0.8 4.7 6 OK 201 44 87 19 Example 3 1.2 1.3 9.6 33 OK 206 48 91 20 Example 4 1.6 0.9 18.5 28 OK 210 56 105 21 Example 5 1.9 1.1 28.7 36 OK 208 65 118 22 Comparative 6 — — — — OK 192 42 73 Example 23 Comparative 7 0.2 0.3 0.6 10 OK 192 42 74 Example 24 Comparative 8 2.3 2.1 58.0 56 NG 180 43 74 Example 25 Comparative 9 0.5 0.3 0.5 63 OK 193 42 74 Example 26 Comparative 10 1.5 1.3 0.6 55 OK 188 41 75 Example 27 Example 11 1.6 0.9 7.5 26 OK 215 48 90 28 Example 12 1.2 1.1 8.1 25 OK 203 45 85 29 Example 13 2.8 1.6 5.1 46 OK 195 43 84

As is clear from Table 2, it is confirmed that the thixomolded articles corresponding to Examples have higher mechanical strength and higher rigidity than the thixomolded articles corresponding to Comparative Examples. In addition, it is confirmed that when the content of SiO₂ is too low, the mechanical strength and the rigidity cannot be sufficiently enhanced, and on the other hand, when the content of SiO₂ is too high, the moldability is poor.

Further, in Comparative Examples in which no binder is added in the manufacturing of the thixomolding material, the mechanical strength and the rigidity of the thixomolded article cannot be enhanced. The reason for the above includes that SiO₂ particles fall off from the magnesium alloy chip and the SiO₂ particles cannot be sufficiently dispersed. 

What is claimed is:
 1. A thixomolding material comprising: a metal body that contains Mg as a main component; and a coating portion that is adhered to a surface of the metal body via a binder and contains SiO₂ particles containing SiO₂ as a main component, wherein an average particle diameter of the SiO₂ particles is less than 20.0 μm, and a mass fraction of the SiO₂ particles in a total mass of the metal body and the SiO₂ particles is 1.0 mass % or more and 40.0 mass % or less.
 2. The thixomolding material according to claim 1, wherein the binder contains waxes.
 3. The thixomolding material according to claim 1, wherein a content of the binder is 0.001 mass % or more and 0.200 mass % or less.
 4. A method for manufacturing a thixomolding material, comprising: a preparation step of preparing a mixture containing a metal body containing Mg as a main component, SiO₂ particles containing SiO₂ as a main component, a binder, and a solvent; a stirring step of stirring the mixture; and a debindering step of removing, by heating the stirred mixture, at least a part of the binder contained in the mixture to obtain a thixomolding material, wherein a mass fraction of the SiO₂ particles in a total mass of the metal body and the SiO₂ particles is 1.0 mass % or more and 40.0 mass % or less, and a content of the binder in the thixomolding material is 0.001 mass % or more and 0.200 mass % or less.
 5. A thixomolded article comprising: a matrix portion that contains Mg as a main component, a first particle portion that is dispersed in the matrix portion and contains Mg₂Si as a main component; and a second particle portion that is dispersed in the matrix portion, present independently of the first particle portion, and contains MgO as a main component, wherein when observing a cross section, a total area fraction of the first particle portion and the second particle portion, in a range of 500 μm square centered on a point at a depth of 1 mm from a surface, is 1.0% or more and 55.0% or less.
 6. The thixomolded article according to claim 5, wherein when observing the cross section, |As−Ac|/Ac is 50.0% or less where As [%] is a total area fraction of the first particle portion and the second particle portion in a range of 500 μm square starting from the surface, and Ac [%] is a total area fraction of the first particle portion and the second particle portion in a range of 500 μm square centered on the point at a depth of 1 mm from the surface.
 7. The thixomolded article according to claim 5, wherein an average particle diameter ratio D(Mg₂Si)/D(MgO) is 0.4 or more and 2.0 or less where D(Mg₂Si) is an average particle diameter of the first particle portion and D(MgO) is an average particle diameter of the second particle portion.
 8. The thixomolded article according to claim 5, wherein a tensile strength is 180 MPa or more and 300 MPa or less, and a Young's modulus is 43 GPa or more and 80 GPa or less. 